Corrective Jaw Surgery for Severe Underbite Correction

Anatomy and Mechanics of a Severe Underbite

A severe underbite is clinically characterised as a skeletal Class III malocclusion, a structural dentofacial deformity in which the lower dental arch and jaw project forward relative to the upper arch. In normal craniofacial anatomy, the maxillary (upper) teeth overlap the mandibular (lower) teeth by approximately two to three millimetres in both horizontal overlap (overjet) and vertical overlap (overbite). When an underbite occurs, this spatial relationship is reversed, creating a negative overjet where the lower incisors sit anterior to the upper incisors.

The underlying skeletal architecture typically involves a discrepancy between the maxilla and the mandible. The deformity may stem from maxillary retrognathism (an underdeveloped or retruded upper jaw), mandibular prognathism (an overdeveloped or protruding lower jaw), or a combination of both. These disproportionate skeletal bases affect not only the dental occlusion but also the support of overlying facial soft tissues, the tone of the perioral musculature, the dimensions of the nasopharyngeal airway, and the position of the tongue within the floor of the mouth.

The biomechanics of mastication are compromised in individuals with severe Class III skeletal patterns. The temporomandibular joints (TMJs) frequently undergo abnormal loading because the mandibular condyles must function within glenoid fossae that are subjected to uneven forces. Additionally, the masticatory muscles, including the masseter, temporalis, and lateral pterygoids, work at a mechanical disadvantage. This altered function leads to inefficient food breakdown, premature dental wear on non-functional cusps, and potential chronic strain across the masticatory apparatus.

Aetiology and Contributing Risk Factors

The primary aetiology of severe skeletal Class III discrepancies is genetic. Polygenic inheritance plays a central role in determining craniofacial morphology, cartilage growth rates at the condylar head, and the timing of sutural fusion in the midface. Patients with a family history of mandibular hyperplasia or midface hypoplasia display a significantly elevated incidence of severe underbites. In some cases, specific congenital syndromes, such as Crouzon syndrome, Apert syndrome, or cleft lip and palate anomalies, restrict midfacial growth, producing marked maxillary retrusion.

Environmental and developmental factors can also influence the severity of an underbite. Chronic paediatric nasal airway obstruction caused by enlarged adenoids, severe allergic rhinitis, or hypertrophic turbinates often forces chronic mouth breathing. This alters tongue posture, lowering it away from the hard palate. Without the physiological expansive pressure of the tongue against the palate, the maxillary arch may become constricted and retrognathic, allowing the mandible to adopt an anteriorly displaced posturing position.

Lifestyle habits and systemic health factors can complicate the presentation and subsequent management of skeletal jaw deformities. In regions where the use of betel quid, areca nut, and smokeless tobacco formulations like gutka or khaini is prevalent, the risk of oral submucous fibrosis (OSMF) rises. OSMF causes severe mucosal stiffness and restricted mouth opening (trismus), which substantially complicates both orthodontic mechanics and orthognathic surgical access. Nutritional deficiencies affecting bone mineral density, such as vitamin D and calcium deficiencies, can also impede skeletal healing following osteotomies.

Clinical Presentation and Functional Symptoms

Severe underbites manifest with distinct functional, dental, and aesthetic characteristics. Functionally, patients experience marked difficulty incising and masticating food. The lack of anterior tooth contact forces individuals to chew predominantly on their posterior molars, which are themselves often in crossbite. This inadequate breakdown of food boluses can lead to gastrointestinal discomfort. Speech disturbances, particularly difficulty articulating sibilant sounds ('s', 'z') and fricative sounds ('f', 'v'), are also common due to the inability of the incisors and lips to form correct phonetic seals.

Dentally, patients frequently present with severe compensatory tilting of the teeth. The lower incisors tend to tip lingually (inwards toward the tongue) while the upper incisors flare labially (outwards toward the lips) as the body attempts to mask the skeletal discrepancy naturally. This dental compensation places roots near the cortical plates of the alveolar bone, increasing the risk of gingival recession, root resorption, and abfraction lesions. Chronic occlusal trauma on mismatched tooth surfaces accelerates enamel wear and destabilises periodontal support over time.

The aesthetic and psychosocial presentation of skeletal Class III discrepancy is often significant. Patients display a concave facial profile, a prominent or pointed chin, a thin upper lip lacking adequate skeletal support, and deficient paranasal fullness that can produce a prematurely aged appearance. Temporomandibular disorders (TMD) may accompany this profile, characterised by joint clicking, crepitus, restricted translation of the condyles, and chronic myofascial pain across the neck, shoulders, and preauricular regions.

Diagnostic Evaluation and Surgical Planning

Diagnosing a severe skeletal underbite requires an extensive orthognathic workup combining clinical assessment with three-dimensional imaging. The clinical evaluation includes a comprehensive assessment of facial symmetry, dental midline relationships, dynamic smile lines, lip competence at rest, and the health of the temporomandibular joints. Oral and maxillofacial surgeons, working alongside orthodontists, analyse the patient's centric relation (CR) compared to maximum intercuspation (MIC) to determine whether a functional slide is exaggerating the underbite.

Radiographic assessment forms the foundation of surgical planning. Lateral cephalometric radiographs are used to carry out detailed hard- and soft-tissue analyses, measuring angles such as SNA, SNB, and the ANB differential to quantify the skeletal discrepancy. Cone Beam Computed Tomography (CBCT) provides high-resolution, three-dimensional views of the facial skeleton, permitting precise mapping of the inferior alveolar nerve within the mandibular canal, assessment of condylar morphology, and evaluation of alveolar bone thickness prior to surgical intervention.

Virtual Surgical Planning (VSP) has largely replaced traditional physical model surgery on mechanical articulators. High-resolution CBCT data are merged with digital optical scans of the dental arches (intraoral scans). Surgeons use computer-aided design and manufacturing (CAD/CAM) software to simulate precise bony movements in three dimensions. Custom surgical splints or patient-specific 3D-printed titanium osteotomy guides and fixation plates are generated to transfer the digital simulation accurately into the operating theatre.

Classification and Staging of Skeletal Discrepancies

Skeletal underbites are classified primarily through cephalometric analyses and clinical occlusal frameworks. Angle's original classification defines a Class III malocclusion by the mesiobuccal cusp of the maxillary first permanent molar occluding posterior to the mesiobuccal groove of the mandibular first permanent molar. However, this dental classification does not differentiate between isolated tooth position anomalies and underlying craniofacial skeletal discrepancies.

Skeletal classification relies on cephalometric landmarks, most notably the ANB angle. A positive ANB angle (typically two to four degrees) indicates a normal Class I skeletal relationship. An ANB angle that falls below zero into negative values designates a skeletal Class III relationship. Severe cases often demonstrate ANB angles beyond negative four or five degrees. The Wits appraisal, which measures the linear distance between perpendicular projections from anatomical points A and B onto the functional occlusal plane, further confirms the true magnitude of the skeletal discrepancy without cranial base length distortions.

Clinical staging also categorises the discrepancy based on the primary anatomical site of deformity. Category 1 involves pure maxillary deficiency with normal mandibular dimensions. Category 2 consists of true mandibular prognathism with a normally positioned maxilla. Category 3 represents a combined, or bimaxillary, deformity involving both maxillary retrusion and mandibular excess. In modern maxillofacial practice, bimaxillary deformities are the most common presentation requiring comprehensive surgical correction.

Treatment Pathways: Camouflage Versus Orthognathic Surgery

When managing a Class III malocclusion, clinicians must choose between orthodontic camouflage and corrective underbite surgery orthognathic intervention. Orthodontic camouflage involves extracting selective premolars or using temporary anchorage devices (TADs) to move teeth without altering the underlying jaw bones. While camouflage can manage mild skeletal discrepancies in patients who decline surgery, attempting it in moderate-to-severe skeletal discrepancies compromises facial aesthetics, risks severe gingival recession from pushing roots through thin cortical plates, and frequently leads to unstable, compromised occlusion.

Corrective jaw surgery represents the definitive, evidence-based gold standard for severe skeletal underbites in skeletally mature patients. Orthognathic surgery physically repositions the jaw bases to correct the core skeletal deformity, improve airway dimensions, balance facial proportions, and establish an ideal functional occlusion. Treatment protocols typically follow a three-stage sequence: preoperative orthodontics (to decompensate the dental arches), orthognathic surgery (to reposition the bones), and postoperative orthodontics (to refine and settle the bite).

In selected centres, a 'surgery-first' approach (SFA) may be considered for patients who meet specific criteria. In SFA, the corrective jaw osteotomy is performed before extensive orthodontic tooth movement. This eliminates the temporary aesthetic worsening seen during traditional preoperative decompensation and harnesses the regional acceleratory phenomenon (a temporary surge in bone turnover) to speed up subsequent orthodontic tooth alignment. However, conventional orthodontic-first pathways remain the standard across the majority of hospital maxillofacial departments.

The Surgical Procedure: Step-by-Step Overview

Underbite surgery orthognathic interventions are performed in an operating theatre under general anaesthesia with nasotracheal intubation, allowing the surgeon unimpeded access to the oral cavity and occlusion. The procedure is performed entirely via intraoral incisions, leaving no external scars on the facial skin. For a combined bimaxillary correction, surgery typically begins in the upper jaw with a Le Fort I osteotomy, where an incision is made in the maxillary vestibular mucosa above the tooth roots to access the underlying maxilla.

The surgeon makes horizontal bone cuts above the roots of the upper teeth, through the lateral maxillary walls, nasal septum, and pterygomaxillary junctions. The maxilla is carefully down-fractured, mobilised, and repositioned forward and vertically as dictated by the preoperative 3D plan. A CAD/CAM surgical splint is placed against the mandibular teeth to index the planned position, and rigid internal fixation is achieved using low-profile titanium miniplates and monocortical screws secured into the zygomaticomaxillary buttresses and piriform rims.

Attention is then directed to the lower jaw, where a Bilateral Sagittal Split Osteotomy (BSSO) is performed. Incisions are made along the anterior border of the mandibular ramus. The bone is split sagitally along the ramus and body, protecting the inferior alveolar nerve running within the mandibular canal. The tooth-bearing segment of the mandible is set back, rotated, or realigned to match the newly positioned maxilla. The split bone segments are secured using titanium bicortical position screws or miniplates. If the chin profile requires refinement, an osseous genioplasty is performed simultaneously before all incisions are closed with absorbable sutures.

Postoperative Recovery, Diet, and Rehabilitation

Immediate recovery occurs in a specialised surgical ward. Most patients stay in hospital for one to two nights for intravenous hydration, pain management, antiemetic therapy, and observation of airway patency. Facial swelling and soft-tissue bruising peak around 48 to 72 hours post-surgery. Rather than having the jaws wired completely shut—a historical method known as intermaxillary fixation—modern rigid internal fixation allows early jaw mobilisation, using light guiding intermaxillary elastics to guide the bite without immobilising the joints.

Dietary compliance is critical to protect the osteotomy sites during bone healing. For the initial two to three weeks, patients must maintain a strict, non-chew liquid and smooth puréed diet. Protein intake, hydration, and micronutrient support are essential. In vegetarian households, deliberate planning is required to ensure adequate protein from pulses, paneer, and enriched broths, alongside calcium and vitamin D supplementation to support bone consolidation. Between weeks four and six, patients transition to a soft diet (foods that can be easily crushed with a fork) before gradually reintroducing normal chewing by week eight to twelve.

Rehabilitation also includes careful oral hygiene and gentle physiotherapy. Because intraoral incisions sit close to the gums, patients use warm saline rinses and prescribed chlorhexidine mouthwashes, alongside ultra-soft surgical toothbrushes to clean tooth crowns without traumatising mucosal suture lines. Active jaw-opening exercises and lateral range-of-motion routines are introduced around week three to four to restore normal mouth opening and prevent TMJ stiffness.

Complications, Risks, and Management Strategies

As with any major maxillofacial procedure, orthognathic surgery carries specific clinical risks. Neurosensory alteration is the most frequent occurrence. The inferior alveolar nerve, which traverses the mandible and provides sensation to the lower lip and chin, is retracted and manipulated during a BSSO. Most patients experience temporary numbness (paresthesia) or altered sensation (dysaesthesia) in the lower lip and chin. Sensation typically recovers gradually over six to twelve months, though a minor proportion of patients may experience permanent, localised sensory deficit.

Skeletal relapse represents another potential complication, wherein the repositioned jaw segments slowly drift toward their original positions. Maxillary advancements generally demonstrate excellent long-term stability, but large mandibular setbacks carry a higher risk of relapse due to tension from the suprahyoid musculature and soft tissues. Rigid fixation with titanium plates, precise preoperative planning, and consistent postoperative elastic wear significantly mitigate this risk. In rare cases of non-union or delayed union, prolonged immobilization or secondary bone grafting may be required.

Other potential surgical risks include postoperative infection, wound dehiscence along incision lines, excessive blood loss, and damage to adjacent tooth roots during osteotomies. Chronic tobacco use, including paan or gutka chewing, significantly impairs microvascular circulation, substantially elevating the risks of wound breakdown, plate infection, and delayed bone healing. Patients must discontinue all tobacco products well prior to surgery and throughout the entire recovery period.

Red Flags and When to Seek Urgent Clinical Attention

While mild oozing, bilateral facial swelling, sore throat, and low-grade postoperative temperature are anticipated following surgery, certain symptoms require immediate emergency evaluation. The most critical red flag is airway compromise. Rapidly increasing swelling in the floor of the mouth, sudden difficulty swallowing liquids, stridor (high-pitched breathing sounds), or any sense of shortness of breath necessitates urgent emergency assessment to secure the airway.

Brisk, active intraoral bleeding or pulsatile epistaxis (nosebleeds) that fails to stop after ten minutes of continuous pressure and head elevation represents another surgical emergency requiring immediate maxillofacial assessment. Persistent high fever above 38.5°C, expanding asymmetrical facial redness, foul-smelling purulent discharge from mucosal incisions, or sudden severe worsening of pain unresponsive to prescribed analgesics point toward acute wound or plate infection.

Sudden, noticeable changes in occlusion—such as upper and lower teeth abruptly failing to meet in the guided splint position, mobility of entire jaw segments, or audible cracking sounds when swallowing—indicate mechanical hardware failure or fixation loosening. Patients experiencing any of these signs must contact their surgical team immediately or attend an emergency department with an on-call oral and maxillofacial surgery service.

Evidence and further reading

The safety, efficacy, and clinical stability of underbite surgery orthognathic interventions are well established in international surgical literature. Guidance from the National Institute for Health and Care Excellence (NICE), the British Association of Oral and Maxillofacial Surgeons (BAOMS), and the American Association of Oral and Maxillofacial Surgeons (AAOMS) confirms that combined surgical-orthodontic treatment provides substantial improvements in masticatory performance, airway volume, and quality of life for severe Class III discrepancies compared to non-surgical alternatives.

Systematic reviews published in the *International Journal of Oral and Maxillofacial Surgery*, the *Journal of Cranio-Maxillo-Facial Surgery*, and the *Cochrane Database of Systematic Reviews* consistently demonstrate high rates of skeletal stability following rigid internal fixation. Long-term studies indicate that while neurosensory disturbance remains a common transient side effect, severe permanent functional disability is rare when surgeries are performed by experienced maxillofacial teams using modern virtual planning protocols.